The present invention relates to an electric capacitor which is provided with liquid cooling. Such capacitors are particularly used in high-frequency industrial generators. They include two capacitor plates which are separated from each other by a dielectric, and particularly by a densely sintered ceramic dielectric body. At least one of the metal layers of one of the capacitor plates is cooled by a cooling liquid, such as water, and the water is contained in contact with the metal plate by a protective jacket.
One such liquid-cooled ceramic capacitor is known from German Published Application DE-OS No. 24 01 802. It includes a protective jacket comprised of a metal cylinder, which is arranged over the capacitor metal plate to be cooled. The jacket avoids acceleration of the migration of silver through the ceramic dielectric and/or the corrosion and erosion of the silver electrodes by the cooling water. The metal cylinder may be comprised, for instance, of brass or copper. The cylinder is soldered over its entire surface to the plate to be cooled by means of a layer having a low-melting alloy of high thermal conductivity and having the desired compatability with the ceramic dielectric with respect to their respective coefficients of expansion. However, mechanical stresses still may occur upon variations in temperatures. Furthermore, either the inside diameter of the ceramic dielectric or the outside diameter of the metal cylinder, which are at the opposed surfaces of the dielectric and the cylinder, must be exactly adapted to each other by grinding. Otherwise, the space presented between the metal cylinder and the tubular dielectric will only be imperfectly filled by the low-melting point solder alloy, which shrinks upon solidification.
For this reason, German Provisional Patent DE-AS No. 12 83 394 shows an electric capacitor with liquid cooling, with a slot-shaped space between the metal cylinder for the cooling liquid and that surface of the ceramic dielectric which is provided with a metal layer of a plate. That space is filled with a material of good electrical and heat conductivity which is in powder form, for instance, powdered silver or silver-coated copper powder.
Both above-described embodiments of electrical liquid-cooled capacitors have a metallic protective jacket which is electrically connected either through a layer of solder metal or by a metal powder to the metal layer of the capacitor plate which is to be cooled. As a result, the cooling liquid is relatively far away from the place where the heat to be dissipated is produced.
German Utility Model DE-GM No. 17 26 891 shows a ceramic capacitor with liquid cooling, in which at least that part of the ceramic dielectric which is covered with electrodes forms or has a cavity to receive the cooling liquid. The publication also describes the self-evident requirement that the metal layers of the plates not be attacked by the cooling liquid. This requirement is satisfied in the embodiment of a capacitor in this DE-GM. The expense of complying with this requirement, however, is very great.
If distilled or demineralized water, whose specific conductance is within the range below 70 .mu.S.cm.sup.-1, is used as the cooling liquid, then it is possible that the metal layer of the outer cooled plate, starting at microscopically small defect points and as a result of the large mechanical forces induced thereby, will be removed in circular areas. This could occur, for instance, in the case of water-cooled pot-shaped capacitors like those known from the catalogue, "Keramische Leistungskondensatoren" ("Ceramic Power Capacitors"), 3A5.78, or from German Provisional Patent DE-AS 10 52 570 of the assignee hereof. The silver particles of the metal layer of the plate are bonded in a frit. Removal of these particles leads to a decrease in the capacitance of the capacitor, until it fails completely as a result of its dropping below the permissible limit of the tolerance of the capacitance.